In the field of telecommunications, laser diodes have been used to generate intensity-modulated light pulses for transmission on optical fibers. Such laser diodes are operated and controlled by a DC bias current and a modulation current to intensity-modulate the laser diode about a DC bias point. However, laser characteristics may change in a number of ways. The operation and optical output power of the laser diodes tend to be sensitive to variations in ambient temperature and age of the component. For example, the laser threshold of the laser diode typically increases with increasing temperature and age. Therefore, to maintain a constant average optical power output, the DC bias current must be increased. In addition, the efficiency of the conversion of signal modulating electrical current to optical power, also known as slope efficiency, tends to decrease with increasing temperature. As a result, to maintain the same slope efficiency or constant peak-to-peak optical signal power, the modulation current must also be increased. FIG. 1 illustrates these known relationships between the laser diode optical power output and temperature.
In most fiber optics applications, both the average optical power output and peak-to-peak optical signal power output the laser diode must be relatively constant despite the ambient temperature reading or the age of the diode. One solution to the temperature variation problem is to attempt to maintain the temperature of the laser diode by insulation and thermo-electric cooling. However, the added cost, power consumption and bulk are unfeasible for high-volume, low-cost applications. Another solution provides expected performance curves based on temperature readings and/or age. Due to variations inherent in laser diodes, the assumption that all laser diodes operate similarly under the same conditions is unrealistic and produces inaccurate performance predictions.
The constant optical average and peak power requirements are even more important in systems where both lower speed digital signals and high speed frequency division multiplexed digitally encoded signals are transmitted via optical fibers. An example of such a system is a fiber-to-the-curb system where both telephony channels and switched video channels are transported. Optical transmission systems carrying switched video signals require a number of parameters to be held constant for satisfactory performance. These parameters include optical average and peak power as well as recovered signal linearity.
When the modulation current approaches or goes below the laser threshold, a phenomenon known as clipping arises. FIG. 2 illustrates clipping of the composite signals with frequency ranging from zero to approximately 600 MHz, for example. It is apparent from FIG. 2 that when clipping occurs, an asymmetric output waveform with severe distortions is produced. When clipping is experienced in a system transporting high frequency signals, such as switched video signals, the carrier-to-distortion ratio of the recovered signal decreases. The result is degraded optical link performance or complete signal loss. The degradation and signal loss are especially traumatic in systems with high efficiency modulation schemes. Therefore, a perfect video image at one instant may deteriorate very quickly the next.
In U.S. Pat. No. 5,268,916 to Slawson et al ., a control system is provided to control the laser output over a wide temperature range by monitoring the output of the laser diode. However, in this circuit, the clipping phenomenon cannot be detected and avoided. Therefore, should the laser high speed source amplitude increase excessively, the modulation current would drive the laser diode below the threshold where it operates as a light emitting diode or LED. This results in severe distortion of both the high speed and low speed signal output signals from the laser diode and the unacceptable degradation or loss of the recovered signal.